The present invention relates to apparatus and methods for treating contaminated soils, particularly those containing hydrocarbon products and hydrocarbon chemicals, such as PCBs, and particularly relates to apparatus and methods for remediating hydrocarbon-contaminated soils in a thermally efficient, environmentally compatible and safe manner. The present invention further relates to a portable soil remediation unit.
Soils are frequently contaminated with hydrocarbon products and this constitutes a highly significant and major pollution problem. The contaminants may range from gasoline through heavy hydrocarbon products and hydrocarbon chemicals, such as PCBs. Various efforts have been directed to remediating the soil and one of the most effective is to thermally treat the soil. However, high cost is an inhibiting factor and, in many cases, is the result of inefficiently designed equipment and limited equipment capacities. For example, a major factor affecting the cost is fuel efficiency, as well as the downstream treatment of the residual gaseous components driven off from the soil. Thermal efficiency is disregarded in many systems. For example, water spray quench systems are frequently used for treating exhaust gas streams, often without regard to heat recovery. Heat recovery in high temperature fume incineration is also frequently ignored. Consequently, the cost for clean-up of contaminated soil ranges typically from $30 a ton to well over $300 per ton, depending on the level of contamination, type of contaminant, type of soil in which the contaminant exists, and overall quantity of the contaminant.
In these prior systems, the basic process for cleaning the soil is to expose it to high temperatures whereby the contaminant is volatilized and subsequently oxidized or processed in a reducing environment to leave a carbon char material in the soil. The temperatures at which the soils must be processed can vary substantially from as low as 300.degree. F. discharge temperature on the soil to over 1,000.degree. F. in order to obtain satisfactory low levels of total residual petroleum hydrocarbons. With these wide-ranging temperatures necessary to clean up a wide variety of contaminants, it is essential to design a remediation system which, not only effectively removes the contaminants, but does so in a thermally and, hence, fuel efficient manner.
Additionally, part of the substantial costs in remediating soil lies in transporting the contaminated soil to the remediation site and transporting the remediated soil back to the original site. Consequently, there is a need for a portable soil remediation unit which will afford similar thermal efficiencies and environmentally safe operations as those remediation units at fixed sites.
In accordance with the present invention, a thermally and fuel efficient system, which may be portable, i.e., transportable on a wheeled vehicle, is provided for cleaning a wide variety of soils contaminated with different hydrocarbon products The system includes essentially a rotary drum having a contaminated soil inlet and a remediated soil outlet at opposite ends of the drum. A burner is provided for flowing hot gases of combustion in counterflow relation to the flow of soil through the drum. The flow of hot combustion gases in contact with the contaminated soil volatilizes the contaminants and substantially remediates the soil. The hot remediated soil is discharged from the drum into one end of a separator, e.g., a baghouse. The hot gases of combustion, now containing the particulates, including dust, from the soil, are passed from the soil inlet end of the drum into the separator adjacent its opposite end. For portability purposes, the drum and the separator, i.e., baghouse, are disposed in side-by-side relation on a wheeled vehicle, for example, a flatbed truck.
In the baghouse, the particulate-laden gases are separated into clean exhaust gas and solid particles, the latter flowing toward the bottom of the baghouse. The remediated soil discharged into the baghouse is conveyed by a screw conveyor to a remediated soil discharge outlet from the baghouse. The particles separated from the particulate-laden gas are combined with the remediated soil en route to the outlet. More particularly, the hot remediated soil in the baghouse is placed in heat exchange relation with the particulate-laden gases whereby the temperature of the particles is elevated to enable release of any residual hydrocarbons remaining on the particles, further cleaning the particles of hydrocarbons. This is accomplished in a thermally efficient manner within the baghouse because the open hopper of the baghouse permits the hydrocarbons to escape from the particles with relative ease. Also, dry-mixing the remediated soil and the clean particles assists to reconstitute the soil to its original gradation. Further, by placing the soil and particles in heat exchange relation within the baghouse, the soil is cooled, rendering it easier to handle and, of course, the heat is thereby used in an efficient manner to burn off the residual hydrocarbons on the particles. This heat transfer relation between the soil and particles also enables the temperature of the baghouse to be considerably lower than otherwise, while still affording satisfactory remediation of the particles. Also, by mixing the remediated soil and particles within the baghouse, dust and other environmentally unsafe particles are prevented from being emitted to the atmosphere as pollution.
In accordance with another feature of the present invention, there is provided a separator within the baghouse for separating the remediated soil into large and small gradations. For example, a vibratory screen may be located inside the baghouse adjacent the inlet for the remediated soil whereby the soil flows onto the screen for separation into large and small gradations. The smaller gradations, i.e., the soil material flowing through the screen, drops from the vibratory screen to the bottom of the baghouse hopper, where it is mixed with the particles separated from the particulate-laden combustion gases. The large gradations may be removed from the baghouse, thereby protecting the downstream conveying and mixing apparatus.
Additionally, in certain types of contaminated soils, large gradations of soil, for example, aggregates of two inches in diameter and greater, may be incompletely remediated in the drum. That is, only the surface of the large aggregates may be fully remediated, with the interior of the aggregate harboring additional contaminants. By passing these large aggregates over the vibratory screen, they are mechanically reduced and the remaining reduced, but still large, aggregates may be removed from the baghouse for reintroduction into the dryer. Thus, the progressively smaller aggregates may be completely remediated.
Further, certain contaminated soils may be so fine or sticky that heat transfer and material flow problems occur within the rotatable drum. In order to balance the gradation for better material flow and heat transfer in the drum, additional rocks or inorganic aggregate of large size may be fed simultaneously with the contaminated soil into the drum. These large size aggregates provide sufficient veiling within the drum, enabling heat transfer from the hot gases of combustion to the large size aggregates, thereby reducing the exit gas temperature of the drum entering the baghouse. Further, the larger aggregates mechanically clean the drum flighting by impact as the large aggregate veils within the drum and impacts the flights. There is also the additional benefit of improved heat transfer between the large aggregates and the contaminated soil within the drum. At the drum discharge and inlet to the baghouse, the large aggregates can be screened from the soil and returned to drum soil feed inlet bin for re-use. These hot, large aggregates therefore preheat the soil prior to its entry into the drum. While the screening can take place outside the drum, it is preferable to screen the materials within the baghouse so that the particulate matter may be captured without polluting the atmosphere.
In a preferred embodiment according to the present invention, there is provided apparatus for remediating contaminated soil comprising a rotary drum having an inlet and an outlet adjacent opposite ends thereof for receiving contaminated soil to be remediated and discharging remediated soil, respectively, a burner for flowing hot gases of combustion through the drum in heat transfer relation with the soil, thereby heating the soil, the drum having a second outlet for discharging gases laden with particulate matter from the soil, a separator having a first inlet in communication with the rotary drum outlet for receiving the hot remediated soil and a second inlet in communication with the second outlet of the drum for receiving the particulate-laden gases whereby the particulate-laden gases and remediated soil are placed in heat transfer relation one with the other within the separator, the separator separating the particulate-laden gases into particles and substantially clean exhaust gas for discharge to the atmosphere. Means are provided in the separator for combining the separated particles and the remediated soil and a discharge from the separator is provided for discharging the combined separated particles and remediated soil.
In a further preferred embodiment according to the present invention, there is provided a method of remediating contaminated soil comprising the steps of heating the soil in a rotating drum by flowing hot gases of combustion through the soil to volatilize the contaminants, flowing the remediated soil into a separator, flowing the particulate-laden gases from the drum into the separator, and in the separator, separating the particulate-laden gases into clean exhaust gases and particles. Further steps include locating the particle-laden gases in heat exchange relation with the remediated soil in the separator to release residual contaminants in the particles of the particulate-laden gases, combining the particles and remediated soil in the separator and discharging the combined particles and remediated soil from the separator.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for remediating contaminated soils in an environmentally safe manner having improved thermal and fuel efficiency whereby capital and operating costs are reduced and substantial portions of the heat in the system are recovered for use in the system, as well as to provide a soil remediation system which is portable for use at the job site.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.